Modern electronic gas turbine engine control systems, like the older hydraulic engine controllers, rely on a variety of measured and calculated flight condition variables in order to properly achieve the desired engine thrust under a wide variety of aircraft operating conditions. Electronic engine controllers achieve the necessary reliability through the use of redundant sensing and calculating subsystems, substituting multiplicity for the prior art simplicity of former hydromechanical controllers.
Certain flight condition variables are absolutely critical to the functioning of any electronic engine controller. Altitude, inlet air total temperature, and aircraft Mach number form the computational basis upon which the controller determines the current available and delivered engine thrust. An absence or an inaccuracy in any one of these critical variables can result in improper engine thrust delivery and/or reduced engine operating efficiency.
Electronic controllers currently in use develop "validated" values of these critical variables by first calculating tentative values of these critical variables from environmental data measured local to an individual gas turbine engine and comparing this tentative value with one or more other tentative values measured and calculated by the instrumentation associated with the particular aircraft in which the engine is mounted. Such control systems require close correlation and cooperation between the airframe electronic instrumentation system and the individual engine controller.
As may be expected, such close cooperation is somewhat difficult to achieve, especially since commercial gas turbine engines and aircraft are not only produced and designed by different manufacturers, but also due to the applicability of an individual engine product line to one or more aircraft which in turn can accept one or more engine lines. Another difficulty may be appreciated by considering that the connection of the engine controller with the aircraft instrumentation system to validate critical flight condition data also requires that each engine-aircraft combination be separately tested and certified by the appropriate government agencies charged with maintaining the safety of the flying public. Still another factor is the increase in vulnerability which accompanies the increased controller dependency on the aircraft instrumentation.
A further problem arises in attempting to fit newer, electronically controlled engines into older aircraft which were not equipped with the necessary electronic instrumentation to provide values of the critical flight condition variables to the engine controllers. Such retrofits currently require costly upgrading of the otherwise satisfactory aircraft instrumentation system to accommodate the electronic engine controllers. What is needed is a validation system for determining critical flight condition data which is operable independently of the aircraft.